This invention relates to a flexible duct for conveying a gaseous flow and, more particularly, to a flexible duct which is structurally useable as the flexible nozzle of a wind tunnel to cause, maintain, and control a uniform gaseous flow over a wide range of nozzle exit velocities from the subsonic to the supersonic. In this regard, it is to be noted and remembered that the exit of a wind tunnel nozzle leads to, and sometimes coincides with, the entrance of the test section of the wind tunnel.
Current practice for contouring the flexible nozzle of the wind tunnel requires the use of a plurality of actuators and/or jacks which are pivotally attached to a flexible wall plate member and to fixed trunnions; position measurement devices for each actuator and/or jack; stress monitoring devices to prevent damage to the flexible wall plate member; and, a complex control system to command movement of each actuator in concert. It is fair and accurate to say that the conventional flexible nozzle assembly requires considerable maintenance to assure accurate contour positioning, and that the probability of mechanical failure to maintain the selected and needed contour positioning is excessively high.
It is here to be noted that an approach which significantly simplifies the problem of controlling the selected contouring of the flexible nozzle is set forth in U.S. Pat. No. 3,443,598 which was issued to Pierce on May 13, 1969. Therein, the flexible wall plate member is supported by multiple pivoted links which are attached to fixed trunnions; translation of the flexible wall plate member and of the test section deforms the flexible plate into predetermined contour shapes; only one actuator per plate member is required; and, as a result, a correspondingly very simple control system is sufficient.
The primary advantages of the approach and apparatus disclosed in the Pierce patent, as compared to the conventional approach and apparatus used to contour flexible wind tunnel nozzles, are: the elimination of the need for stress monitoring equipment; and, greatly reduced maintenance requirements. However, the approach and apparatus disclosed in the Pierce patent are deficient in three major aspects, namely: (1) the test section movement, as Mach number is changed, results in difficulties with respect to test article support within the test section; (2) the physical limits on travel of the flexible wall plate member restricts the range of Mach numbers that are obtainable; and, (3) the inherent small angle, between the flexible wall plate member and the links at low Mach number contours, results in excessive deflection under normal operating conditions. Elimination of these deficiencies is the basis of the instant invention, which constitutes a significant advance in the state-of-the-art.